Accelerator Magnet Quench Heater Technology and Quality Control Tests for the LHC High Luminosity Upgrade

The High Luminosity upgrade of the Large Hadron Collider (HL-LHC) foresees the installation of new superconducting Nb$_{3}$Sn magnets. For the protection of these magnets, quench heaters are placed on the magnet coils. The quench heater circuits are chemically etched from a stainless steel foil that is glued onto a flexible Polyimide film, using flexible printed circuit production technology. Approximately 500 quench heaters with a total length of about 3000 m are needed for the HL-LHC magnets. In order to keep the heater circuit electrical resistance in acceptable limits, an approximately 10 µm-thick Cu coating is applied onto the steel foil. The quality of this Cu coating has been found critical in the quench heater production. The work described in this thesis focuses on the characterisation of Cu coatings produced by electrolytic deposition, sputtering and electron beam evaporation. The quality of the Cu coatings from different manufacturers has been assessed for instance by ambient temperature electrical resistance measurements, Residual Resistivity Ratio (RRR) measurements, adhesion tests. The tested electrolytic Cu coatings with Ni interlayer adhere well on the steel substrate, and they have the required room temperature and 4.2 K electrical resistivity. Electron beam evaporated Cu coatings can exhibit the required adhesive strength and RRR. The quality of the sputter coated Cu layers is strongly dependent on the manufacturer and coating parameters. A procedure for the efficient testing of the Cu coating thickness distribution on the quench heater base material and on the finished heaters, based on 4-point electrical resistance measurements, has been developed. The influence of the electrically conductive 304L steel and interlayer substrate on the Cu coating thickness determination by resistance measurements has been studied and correction factors have been introduced. To evaluate the adhesion of the Cu coating to the 304L surface, a number of adhesion tests have been researched and applied to the Cu coating production samples. For the 11 T dipole Nb$_{3}$Sn HL-LHC magnets interlayer quench heaters are developed that must withstand the 650 °C – 50 h coil reaction heat treatment. In collaboration with the CERN thin film laboratory a study was undertaken to determine the minimum required thickness of an efficient diffusion barrier coating that can prevent Ni diffusion from the substrate into to Cu coating and RRR degradation.